Nitride semiconductor light emitting device chip

ABSTRACT

In a nitride semiconductor light emitting device chip, a mask pattern on a nitride semiconductor substrate ( 101 ) is formed of a growth inhibiting film on which a nitride semiconductor layer is hard to grow. There are a plurality of windows unprovided with the growth inhibiting film. There are at least two different widths as mask widths each between the adjacent windows. The mask pattern includes a mask A group (MAG) and mask B groups (MBG) arranged on respective sides of the mask A group. A mask A width in the mask A group is wider than a mask B width in the mask B group. The nitride semiconductor light emitting device chip further includes a nitride semiconductor underlayer ( 102 ) covering the windows and the mask pattern, and a light emitting device structure having a light emitting layer ( 106 ) including at least one quantum well layer between an n type layer ( 103-105 ) and a p type layer ( 107-110 ) over the underlayer. A current-constricting portion (RS) through which substantial current is introduced into the light emitting layer is formed above mask A.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/JP01/11605 filed on Dec. 27, 2001which claims priority to Japanese Patent Application No. 2001-000069filed on Jan. 4, 2001, and Japanese Patent Application No. 2001-052175filed on Feb. 27, 2001, the contents of each of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to nitride semiconductor light emittingdevices, and more particularly to improvement in their light-emittinglifetimes, luminous intensities, operating voltages and yields.

BACKGROUND ART

Jpn. J. Appl. Phys., Vol. 39 (2000), pp. L647-650 teaches a way ofimproving an output and a lifetime of a nitride semiconductor laserdevice, wherein a GaN underlayer is grown to cover a mask pattern ofuniform SiO₂ stripes formed over an entire region of one main surface ofa GaN substrate and windows unprovided with the SiO₂ masks, and thenitride semiconductor laser device is formed on the GaN underlayer.

According to the article, threading dislocations in the GaN underlayerdecrease in a region above the SiO₂ mask, and then the output and thelifetime of the laser device can be improved by utilizing the underlayerregion having such a small threading dislocation density.

However, it is still desired to improve the lasing lifetime even in thenitride semiconductor laser device disclosed in the above-describedarticle Jpn. J. Appl. Phys., Vol. 39 (2000), pp. L647-650.

Incidentally, there are different two ways to provide a pair ofelectrodes to a semiconductor laser chip. In the first way, twoelectrodes (a p-side electrode and an n-side electrode) opposite to eachother are provided on a front side of a semiconductor laser devicestructure formed on a substrate and on a back side of the substrate,respectively. Hereinafter, such an arrangement of electrodes is called“counter electrodes arrangement”. In the second way, a p-side electrode(or an n-side electrode) is provided on a front side of a semiconductorlaser device structure formed on a substrate, and a portion of an n typelayer (or a p type layer) is exposed by reactive ion etching on thefront side of the device structure to provide an n-side electrode (or ap-side electrode) on the exposed region. That is, both the p-sideelectrode and the n-side electrode are provided on the same side of thesubstrate. Hereinafter, such an arrangement of electrodes is called“one-side electrodes arrangement”.

In comparison between the counter electrodes arrangement and theone-side electrodes arrangement, while it is necessary to secure boththe regions for forming the p-side and n-side electrodes on the frontside of the substrate in the one-side electrodes arrangement, the backside of the substrate can be used as the electrode formation region inthe counter electrodes arrangement. That is, in the laser chip havingthe counter electrodes arrangement, the substrate area can be used moreefficiently, allowing reduction of the chip size. Although formation ofa laser chip having the one-side electrodes arrangement requiresreactive ion etching of a portion of the semiconductor laser devicestructure, such a complicated process is unnecessary to form a laserchip having the counter electrodes arrangement.

In the nitride-based semiconductor laser device having the counterelectrodes arrangement, however, its threshold voltage is liable toincrease considerably compared to the case of the laser device havingthe one-side electrodes arrangement. Thus, it is common to employ theone-side electrodes arrangement in a semiconductor laser device.

Under these circumstances, it is also desired to decrease the thresholdvoltage of the semiconductor laser device having the counter electrodesarrangement in order to promote utilization of the semiconductor laserdevice with the counter electrodes arrangement which has the advantagesof reduced chip size and simplified manufacturing process.

In view of the foregoing, a primary object of the present invention isto further improve nitride semiconductor light emitting devices in theirlight-emitting lifetimes, luminous intensities, yields and others.Another object of the present invention is to further improve nitridesemiconductor laser devices having the counter electrodes arrangement intheir threshold voltages and others.

DISCLOSURE OF THE INVENTION

A nitride semiconductor light emitting device chip according to anaspect of the present invention includes a mask substrate including amask pattern formed on a main surface of a nitride semiconductorsubstrate. The mask pattern is formed of a growth inhibiting film whichsuppresses epitaxial growth of a nitride semiconductor layer thereon,and there are a plurality of windows unprovided with the growthinhibiting film. There are at least two different widths of masks eachbetween the windows adjacent to each other, and the mask patternincludes a mask A group and mask B groups. The mask B groups arearranged on respective sides of the mask A group, and a mask A width inthe mask A group is set wider than a mask B width in the mask B group.The nitride semiconductor light emitting device chip further includes anitride semiconductor underlayer covering the windows and the maskpattern, and a light emitting device structure having a light emittinglayer including a quantum well layer (or a quantum well layer and abarrier layer in contact with the quantum well layer) between an n typelayer and a p type layer over the underlayer. A current-constrictingportions through which substantial electric current is introduced intothe light emitting layer is formed above mask A.

The current-constricting portion is preferably formed more than 2 μmaway from the center of the width of mask A. Further, thecurrent-constricting portion is preferably formed above a region acrossmask A and window A. Still further, the current-constricting portion ispreferably included completely in a region right above mask A.

A window A width in the region of the mask A group is preferably setnarrower than a window B width in the region of the mask B group. Themask A width is preferably in a range of 10-20 μm, and the window Awidth is preferably in a range of 2-10 μm. The mask B width ispreferably in a range of 2-10 μm, and the window B width is preferablyin a range of 5-40 μm. Mask A is preferably formed in a stripe shapehaving its longitudinal direction parallel to either a <1-100> directionor a <11-20> direction of the nitride semiconductor substrate.

The nitride semiconductor underlayer preferably includes Al_(x)Ga_(1−x)N(0.1≦x≦0.15) or In_(x)Ga_(1−x)N (0.1≦x≦0.18). Further, the quantum welllayer preferably contains at least one of As, P and Sb.

A nitride semiconductor laser chip according to another aspect of thepresent invention includes a mask substrate including a mask groupformed on a region of a main surface of a nitride semiconductorsubstrate having a polarity of n type or p type. The mask group includesa plurality of masks and a plurality of windows arranged alternatelywith each other. Each of the masks is formed of a growth inhibiting filmsuppressing epitaxial growth of a nitride semiconductor layer thereon,and each of the windows is unprovided with the growth inhibiting film.The nitride semiconductor laser chip further includes a nitridesemiconductor underlayer having a polarity and covering an entire regionincluding the mask group over the mask substrate, and a light emittingdevice structure having a light emitting layer including a quantum welllayer (or a quantum well layer and a barrier layer in contact with thequantum well layer) between an n type layer and a p type layer over theunderlayer. A current-constricting portion through which substantialelectric current is introduced into the light emitting layer is formedabove the mask group, and electrodes are formed on a back side of themask substrate and a front side of the light emitting device structure,respectively.

In the width direction orthogonal to the length direction of a resonatorof the laser chip, the width of the mask group is preferably less than50% of the width of the laser chip. Further, the mask preferably has awidth in a range of 10-20 μm, and the window preferably has a width in arange of 2-10 μm.

The nitride semiconductor underlayer preferably includes Al_(x)Ga_(1−x)N(0.01≦x≦0.15) or In_(x)Ga_(1−x)N (0.01≦x≦0.18). Further, the quantumwell layer preferably contains one of As, P and Sb.

The nitride semiconductor light emitting device chips of the presentinvention as describe above can favorably be used in optical apparatusesand light emitting apparatuses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view illustrating an example of anitride semiconductor laser device chip according to the presentinvention.

FIG. 2 is a schematic diagram showing an upper surface and a sidesurface of a filmed mask substrate usable in the present invention.

FIG. 3 is a schematic cross sectional view illustrating another exampleof the nitride semiconductor laser device chip according to the presentinvention.

FIG. 4 is a schematic cross sectional view illustrating yet anotherexample of the nitride semiconductor laser device chip according to thepresent invention.

FIGS. 5A and 5B are schematic cross sectional views illustrating growthof a nitride semiconductor film on a mask substrate.

FIG. 6A is a schematic cross sectional view illustrating a masksubstrate usable in the present invention, and FIG. 6B is a schematiccross sectional view illustrating a filmed mask substrate formed fromthe mask substrate of FIG. 6A.

FIG. 7 is a schematic cross sectional view illustrating a furtherexample of the nitride semiconductor laser device chip according to thepresent invention.

FIG. 8A is a schematic cross sectional view illustrating a nitridesemiconductor laser device chip having a ridge stripe structure, andFIG. 8B is a schematic cross sectional view illustrating a nitridesemiconductor laser device chip including a current-blocking layer.

FIGS. 9A-9C are top plan views of mask substrates usable in the presentinvention, among which FIG. 9A shows a mask substrate where a maskpattern including stripes in two directions orthogonal to each other isarranged at each side of a mask group A, FIG. 9B shows a mask substratewhere a mask pattern including stripes in two directions at 60 degreeswith each other is arranged at each side of a mask group A, and FIG. 9Cshows a mask substrate where a mask pattern including stripes in threedirections at 60 degrees with each other is arranged at each side of amask group A.

FIG. 10 is a schematic cross sectional view illustrating still anotherexample of the nitride semiconductor laser device chip according to thepresent invention.

FIG. 11 is a schematic diagram illustrating an upper surface and a crosssection of a mask substrate usable in the present invention.

FIG. 12 is a schematic cross sectional view illustrating a filmed masksubstrate usable in the present invention.

FIG. 13 is a schematic cross sectional view illustrating a nitridesemiconductor laser chip formed on a mask substrate according to thepresent invention.

FIG. 14 is a graph illustrating the relation between the coverage ratioof the mask group and the threshold voltage in the present invention.

FIG. 15 is a schematic cross sectional view illustrating an embodimentof the nitride semiconductor laser device chip including acurrent-blocking layer.

FIG. 16 is a schematic block diagram illustrating an example of anoptical apparatus which includes an optical pickup apparatus utilizingthe nitride semiconductor laser chip according to the present invention.

In the drawings of the present application, the same or correspondingportions are denoted by the same reference characters. In the drawingsof the present application, the dimensional relationships in length,width, thickness and others are arbitrarily changed for the purposes ofclarification and simplification of the drawings, rather than showingthe actual dimensional relationships.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, several terms are defined before description of variousembodiments of the present invention.

A “nitride semiconductor substrate” refers to a substrate includingAl_(x)Ga_(y)In_(z)N (0≦x≦1; 0≦y≦1; 0≦z≦1; x+y+z=1), wherein less thanabout 10% of the nitrogen element included in the nitride semiconductorsubstrate may be substituted with at least one of As, P and Sb (providedthat the hexagonal crystal system of the substrate is maintained).Further, the nitride semiconductor substrate may be doped with at leastone impurity selected from Si, O, Cl, S, C, Ge, Zn, Cd, Mg and Be.

A “nitride semiconductor substrate having a polarity” refers to anitride semiconductor substrate which has a polarity of n type or ptype.

A “mask group” refers to a region where a plurality of masks formed ofgrowth inhibiting films and a plurality of windows are arrangedalternately.

A “growth inhibiting film” refers to a film on which epitaxial growth ofa nitride semiconductor layer is unlikely to occur. For example, thegrowth inhibiting film can be formed with a dielectric film, morespecifically with SiO₂, SiN_(x), Al₂O₃ or TiO₂.

A “window” refers to a portion where an underlayer is exposed, notcovered with a mask pattern formed of growth inhibiting films.

A “mask substrate” refers to a substrate where a mask pattern of growthinhibiting films is formed on at least a partial region of a mainsurface of a nitride semiconductor substrate (see FIGS. 6A and 11).

A “nitride semiconductor underlayer” refers to a nitride semiconductorfilm which covers an entire region including the mask group(s) over themask substrate and includes Al_(x)Ga_(y)In_(z)N (0≦x≦1; 0≦y≦1; 0≦z≦1;x+y+z=1). Similarly as in the case of the nitride semiconductorsubstrate, less than about 10% of the nitrogen element included in thenitride semiconductor underlayer may be substituted with at least one ofAs, P and Sb. Further, the underlayer may be doped with at least oneimpurity selected from Si, O, Cl, S, C, Ge, Zn, Cd, Mg and Be.

A “filmed mask substrate” refers to an improved substrate which includesa nitride semiconductor underlayer covering an entire region includingthe mask group(s) over the mask substrate (see FIGS. 6B and 12).

A “light emitting layer” refers to a layer which includes at least onequantum well layer or a plurality of barrier layers stacked alternatelywith the quantum well layers and which can emit light. The lightemitting layer of a single quantum well structure is formed of only asingle well layer or formed of stacked layers of barrier layer/welllayer/barrier layer. The light emitting layer of a multiple quantum wellstructure of course includes a plurality of well layers and a pluralityof barrier layers stacked alternately with each other.

A “light emitting device structure” refers to a structure whichincludes, in addition to a light emitting layer, an n type layer(s) anda p type layer(s) sandwiching the light emitting layer.

A “current-constricting portion” refers to a portion through whichsubstantial electric current is introduced into a light emitting layervia a p type layer or an n type layer, and a “current-constrictingwidth” refers to a width of the relevant portion. In the case of asemiconductor laser having a ridge stripe structure, for example, thecurrent-constricting portion corresponds to a ridge stripe portion (RS)shown in FIG. 8A, and the current-constricting width corresponds to aridge stripe width. In the case of a semiconductor laser having acurrent-constricting layer, the current-constricting portion correspondsto a portion (CS) sandwiched between two current-blocking layers (CB)shown in FIG. 8B, and the current-constricting width corresponds to awidth between the current-blocking layers.

[First Embodiment]

In the present invention, a nitride semiconductor light emitting devicestructure is formed on a nitride semiconductor underlayer which covers amask substrate having masks of growth inhibiting films and windowsuncovered with the mask over a nitride semiconductor substrate. A maskwidth and/or a window width is changed appropriately within one nitridesemiconductor device, so that it becomes possible to provide a nitridesemiconductor light emitting device improved in its light-emittinglifetime, luminous intensity and others, and to suppress cracks liableto occur in the light emitting device.

The effects of the present invention by changing the mask width and/orthe window width appropriately in one nitride semiconductor lightemitting device chip can be obtained only in the case that a substrateincluded in the mask substrate is a nitride semiconductor. A nitridesemiconductor underlayer grown on a mask substrate including a substrateother than the nitride semiconductor substrate (hereinafter, referred toas a “non-nitride substrate”) suffers strong stressed strain compared tothe case of using the nitride semiconductor substrate, since differencein thermal expansion coefficient between the non-nitride substrate andthe nitride semiconductor underlayer is considerably greater than thatbetween the nitride semiconductor substrate and the nitridesemiconductor underlayer. As such, in the case that the nitridesemiconductor substrate is replaced with a non-nitride substrate, evenif the mask and the window are formed with appropriate widths accordingto the present invention, crystal strain within the nitridesemiconductor film (including the light emitting device structure formedon the nitride semiconductor underlayer) covering the mask substrate isnot similarly alleviated as in the case of the present invention. Inaddition, the effect of suppressing cracks which improves yield of thelight emitting devices, the effect of reducing the threshold currentdensity and the effect of reducing the threshold voltage are notsimilarly expected as in the case of the present invention.

(Varied Mask Widths)

The effect of varying the mask widths is described with reference to theschematic cross section of FIG. 1. In the nitride semiconductor laserdevice shown in this figure, a mask A group (MAG) including periodicallyprovided stripe masks A (MA) of growth inhibiting masks and a mask Bgroup (MBG) including a plurality of similar masks B (MB) are formed ona nitride semiconductor substrate 101. An underlayer 102 of a nitridesemiconductor film, n type layers 103-105, a light emitting layer 106,and p type layers 107-110 are successively crystal-grown to cover themask groups. The mask B groups are formed on respective sides of themask A group, and a ridge stripe portion (RS) of the laser device isformed above mask A (MA). Herein, a width of the window between adjacentmasks A is called a window A width (WAW), and a width of the windowbetween adjacent masks B is called a window B width (WBW).

In FIG. 1, the window A width and the window B width were set equal toeach other to study the effect of varied mask widths. The mask B widthwas set narrower than the mask A width. The nitride semiconductor laserdevice of FIG. 1 can be produced in a similar manner as will bedescribed later in the second embodiment.

According to a result of the inventors' study, lasing lifetime tended toextend in the case that the ridge stripe portion of the nitridesemiconductor laser device was formed above one mask A and that thecenter line (MAC in FIG. 1) of the mask A width did not cross the ridgestripe portion (RS). More detailed investigation revealed that thelasing lifetime began to extend remarkably when the ridge stripe portionwas formed above mask A and then a distance from the center line of themask A to a closer side edge of the ridge stripe portion became morethan about 2 μm. In particular, the lasing lifetime became the longestin the case that the ridge stripe portion was completely included withinthe region above mask A, and the lasing lifetime was improved even inthe case that the ridge stripe portion was formed above a regioncrossing the boundary between mask A and window A. The latter case ispreferable in that the threshold current density is decreased by aboutsome %. It is considered that the lasing lifetime extends becausecrystal strain of the nitride semiconductor layer is alleviated abovethe mask than above the window.

From the standpoint of a long lasing lifetime, the mask A width (MAW) ispreferably more than 10 μm and less than 20 μm, and more preferably morethan 13 μm and less than 20 μm. If the mask A width is less than 10 μm,considerable elongation of the lasing lifetime may not be expected. Whenthe mask A width becomes more than 13 μm, the lasing lifetime begins toextend remarkably (e.g., the lasing lifetime becomes more than about15000 hours with laser output of 30 mW at an ambient temperature of 60°C.). On the other hand, the lasing lifetime began to decrease graduallywhen the mask A width exceeded 20 μm, presumably because of thefollowing reason. When the mask width is within the range from 10 μm to20 μm, the crystallographic axis of the nitride semiconductor layerabove the mask is essentially somewhat inclined with respect to adirection perpendicular to the main surface of the substrate. When themask width exceeds 20 μm, the inclination of the crystallographic axisbecomes so large that it cannot be neglected with respect to lasing. Inaddition, when the mask width exceeds 20 μm, lateral crystal growthabove the mask becomes insufficient, leading to formation of a voidabove the center of the mask width.

In view of the result of the study regarding the mask A width asdescribed above, only the mask A group having a mask A width set withinthe range of 10-20 μm was formed on the nitride semiconductor substrate,and a nitride semiconductor laser device was formed using the same. Withsuch a nitride semiconductor laser device, however, the rate ofdefective devices was high. The inventors found, through detailedexamination of the defective device chip, that many cracks occurredacross the ridge stripe portion.

Conventionally, it has been considered that cracks hardly occur in anitride semiconductor laser device when it is formed on a GaN substrate.When only the mask A group was practically formed on a GaN substrate anda nitride semiconductor laser device was grown on a nitridesemiconductor underlayer covering the mask A group, however, many cracksoccurred in the laser device. This is presumably because the nitridesemiconductor laser device is formed of a stacked structure of variouskinds of layers (e.g., an AlGaN layer has a small lattice constantcompared to that of a GaN layer, and an InGaN layer has a large latticeconstant compared to that of the GaN layer). Another conceivable reasonis that a GaN substrate obtained by currently available technique haslatent residual strain in itself.

In view of the foregoing, in the present embodiment, the mask B groupsare arranged on respective sides of the mask A group that is arrangedapproximately beneath the ridge stripe portion. Here, the mask B groupon the either side of the mask A group is provided within the samenitride semiconductor laser device chip (see FIG. 1), and the mask Bwidth is set narrower than the mask A width. By doing so, it is possibleto realize a long lasing lifetime as well as to suppress occurrence ofcracks across the ridge stripe portion.

The crack suppressing effect of the present embodiment is explained indetail with reference to FIG. 2. In this figure, a filmed mask substrate100 is shown in top plan view and in side view, where a mask A group(MAG) and mask B groups (MBG) are formed on a nitride semiconductorsubstrate 101, and an underlayer 102 of nitride semiconductor film iscrystal-grown to cover the mask groups. Here, the mask B groups arearranged on respective sides of the mask A group, and the mask B width(MBW) is set narrower than the mask A width (MAW). Masks MB1, MB2 andMB3 shown in FIG. 2 belong to the mask B group, which are numbered forconvenience of the explanation. Further, a region A shown in FIG. 2illustrates a range of the nitride semiconductor underlayer stackedabove the mask A group.

Referring to FIG. 2, progress of a crack CA having occurred at thesurface of the nitride semiconductor underlayer was blockedapproximately above the center of the width of mask MB1 (broken line BLain FIG. 2). Progress of a crack CB having extended across mask MB1 andreached mask MB2, was blocked approximately above the center of thewidth of mask MB2 (broken lines BLb in FIG. 2). Similarly, progress of acrack CC was blocked approximately above the center of the width of maskMB3 (broken line BLc in FIG. 2). As such, progress of the cracksapproaching region A from the outside in FIG. 2 is blocked by the mask Bgroup (MBG), and as a result, the rate of defective nitridesemiconductor laser devices can be reduced if the ridge stripe portionis formed within region A in every laser device.

This is presumably because while the mask A width cannot be narrowedfreely from the standpoint of a long lasing lifetime, narrowing the maskB width (MBW) than the mask A width (MAW) causes the following effects.Firstly, it is considered that inclination of the crystallographic axisin the nitride semiconductor layer can be made smaller above mask B thanabove mask A by narrowing the mask B width than the mask A width.Secondly, it is considered that the narrowed mask width causes reductionin size of a void liable to occur above the mask.

On the other hand, if the crystallographic axis orientation of thenitride semiconductor layer above the mask is degraded or a large voidoccurs above the mask, then the mask may become a new source ofoccurrence of a crack.

According to a result of the inventors' study, the above-described cracksuppressing effect is most effective when the mask B width is more than2 μm and less than 10 μm. The crack suppressing effect is halved unlessthe mask B width is set narrower than the mask A width.

Although the effect of varied mask widths has been explained taking thenitride semiconductor laser device chip having a ridge stripe structure(see FIG. 8A) as an example, the effect of varied mask widths issimilarly obtained also in a nitride semiconductor laser device chiphaving a current-blocking layer (see FIG. 8B). In the case of thenitride semiconductor laser device chip having the current-blockinglayer, the above-described ridge stripe portion (RS) corresponds to aportion (CS) sandwiched between two current-blocking layers (see FIGS.8A and 8B). More generally, the ridge stripe portion and the portionsandwiched between the two current-blocking layers each correspond to acurrent-constricting portion through which substantial electric currentis introduced via a p type layer or an n type layer into a lightemitting layer and which contributes to emission of light.

It is needless to say that the mask groups on the nitride semiconductorsubstrate are not limited to two kinds of mask groups, mask A group andmask B group, but more than two kinds of mask groups may be provided asdesired.

(Varied Window Widths)

The effect of varying the window widths is described with reference tothe schematic cross sections in FIGS. 3 and 4. In the nitridesemiconductor laser device chip shown in FIG. 3, an n electrode 111 anda p electrode 112 are formed on the same side with respect to a nitridesemiconductor substrate 101. A mask A group (MAG) and mask B groups(MBG) are formed on nitride semiconductor substrate 101, and anunderlayer 102 of nitride semiconductor film, n type layers 103-105, alight emitting layer 106, and p type layers 107-110 are successivelycrystal-grown to cover the mask groups. The mask A group (MAG) isarranged approximately beneath a ridge stripe portion (RS), and the maskB groups (MBG) are arranged on respective sides of the mask A group.

In FIG. 3, the mask A width (MAW) and the mask B width (MBW) have beenset equal to each other in the range of 10-20 μm, to study the effect ofvaried window widths. The window B width has been set wider than thewindow A width. The nitride semiconductor laser device chip shown inFIG. 3 may be made in a similar manner as will be described later in thesecond embodiment.

According to a result of the inventors' study, the lasing lifetime tendsto extend as the window A width (WAW) in the mask A group (AG) beneaththe ridge stripe portion (RS) becomes narrower. This is presumablybecause the crystal-grown nitride semiconductor layer exhibits a smallereffect of alleviating crystal strain in a region above the window widththan in another region above the mask. That is, narrowing the windowwidth leads to an increased mask occupying rate per unit area, which inturn increases the nitride semiconductor layer region alleviated incrystal strain. More detailed investigation from the standpoint of anextended lasing lifetime has revealed that the window A width ispreferably greater than 2 μm and less than 10 μm, and more preferablygreater than 2 μm and less than 6 μm. When the window A width becameless than 10 μm, the lasing lifetime began to extend, and the elongationof the lasing lifetime was remarkable when the window A width becameless than 6 μm (e.g., the lasing lifetime was more than about 15000hours with a laser output of 30 mW at an ambient temperature of 60° C.).On the other hand, when the window A width becomes less than 2 μm, itbegins to be difficult to completely cover the masks flatly, even if thenitride semiconductor underlayer covering the masks is grown thick.

In view of the result of the study regarding the window A width asdescribed above, only the mask A group having a window A width set inthe range of 2-10 μm was formed on the nitride semiconductor substrate,and a nitride semiconductor laser device was produced utilizing thesame. The nitride semiconductor laser device thus produced, however,exhibited a high threshold current density.

Then, in the present invention, mask B groups (MBG) are arranged onrespective sides of the mask A group (MAG) which is approximatelybeneath the ridge stripe portion (RS). Here, the mask B group on theeither side of the mask A group is provided within the same nitridesemiconductor laser device chip (see FIG. 3), and the window B width isset wider than the window A width as described above.

By doing so, it has been found that the threshold current density of thelaser device can be decreased while realizing a long lasing lifetime.According to a result of more detailed investigation of the inventors,the window B width (WBW) is preferably greater than 5 μm and less than40 μm, and more preferably greater than 11 μm and less than 30 μm.However, it is necessary to maintain the relation that the window Bwidth is wider than the window A width. The effect of reducing thethreshold current density began to appear when the window B width becamemore than about 5 μm, and the effect of reducing the threshold currentdensity by more than about 3% began to appear when the window B widthbecame more than 11 μm. There is no upper limit of the window B widthfrom the standpoint of the threshold current density; i.e., the widerwindow B width is more preferable. When the window B width is too wide,however, it becomes difficult to decrease the crystal strain of thenitride semiconductor laser device. Thus, the window B width ispreferably less than 40 μm, and more preferably less than 30 μm, fromthe standpoint of alleviation of the crystal strain.

A nitride semiconductor laser device chip having an n electrode formedon a back side of the nitride semiconductor substrate and having a pelectrode formed on a p type layer is now described with reference toFIG. 4. In this nitride semiconductor laser device, a mask A group (MAG)and mask B groups (MBG) are formed on a nitride semiconductor substrate101, and an underlayer 102 of nitride semiconductor film, n type layers103-105, a light emitting layer 106, and p type layers 107-110 aresuccessively crystal-grown to cover the mask groups. A ridge stripeportion (RS) is formed above the mask A group AG), and the mask B groups(MBG) are arranged on respective sides of the mask A group.

In FIG. 4, the mask A width and the mask B width were set equal to eachother within a range of 10-20 μm in order to study the effect of variedwindow widths. The window B width was set wider than the window A width.The nitride semiconductor laser device shown in FIG. 4 may be producedin a similar manner as will be described later in the second embodiment.

Similarly as described before, the window A width (WAW) of the mask Agroup located beneath the ridge stripe portion is preferably greaterthan 2 μm and less than 10 μm, and more preferably greater than 2 μm andless than 6 μm, from the standpoint of a long lasing lifetime.

According to a result of the inventors' study, it has been found thatthe lasing threshold voltage in the nitride semiconductor laser deviceis reduced by about 0.3 V to about 1 V when the mask B groups having thewindow B width (WBW) wider than the window A width are arranged onrespective sides of the mask A group as shown in FIG. 4, compared to thecase of including only the mask A group.

According to a result of detailed investigation of the inventors, thewindow B width (WBW) is preferably greater than 5 μm and less than 40μm, and more preferably greater than 11 μm and less than 30 μm,similarly as described before. However, it is necessary to maintain therelation that the window B width is wider than the window A width. Whenthe window B width is greater than about 5 μm, the effect of reducingthe threshold voltage begins to appear, and the effect of reduction ofmore than about 0.5 V begins to be obtained when the window B width isgreater than 11 μm. There is no upper limit of the window B width fromthe standpoint of the threshold voltage; i.e., the wider window B widthis more preferable. When the window B width is too wide, however, thecrystal strain of the nitride semiconductor laser device is less likelyto be reduced. Thus, the window B width is preferably less than 40 μm,and more preferably less than 30 μm, from the standpoint of alleviationof the crystal strain.

Although the effect of varied window widths has been explained takingthe nitride semiconductor laser device chip having a ridge stripestructure (see FIG. 8A) as an example, the effect of varied windowwidths is expected also in the nitride semiconductor laser device chiphaving a current-blocking layer (see FIG. 8B). Further, it is needlessto say that, not limited to the two kinds of mask groups of mask A groupand mask B group, more than two kinds of mask groups may be provided onthe nitride semiconductor substrate as desired. It is also needless tosay that the feature of varied window widths may be combined with thefeature of varied mask widths described before.

(Longitudinal Direction of Stripe Mask)

The longitudinal direction of the stripe mask formed on the nitridesemiconductor substrate having a main surface of a {0001} C. plane wasmost preferably in parallel with a <1-100> direction, and nextpreferably in parallel with a <11-20> direction. Even when thelongitudinal direction of the mask made an angle of the order of within±5° to such a specific crystal direction in the {0001} C. plane, it didnot cause any substantial effect.

Forming the masks along the <1-100> direction of the nitridesemiconductor substrate is advantageous in that the crack suppressingeffect is remarkable. When the masks formed along this direction arecovered with a nitride semiconductor film, the growing film formsprimarily {11-20} facet planes on the masks and then covers them (seeFIG. 5A). The {11-20} facet plane (FP1) is perpendicular to the mainsurface of substrate 101, and the mask (M) is formed of a growthinhibiting film suppressing epitaxial growth thereon. Thus, the nitridesemiconductor regions (RA) and (RB) shown in FIG. 5A come into contactwith each other only in their {11-20} facet planes (FP) thereby coveringthe masks. Accordingly, the crack which appeared in the nitridesemiconductor region (RA) in FIG. 5A was hard to propagate into thenitride semiconductor region (RB). This feature of the <1-100> directioncan be combined with the feature of the mask B group to cause a moreenhanced crack suppressing effect.

On the other hand, formation of the masks along the <11-20> direction ofthe nitride semiconductor substrate is advantageous in that, when themasks are covered with a nitride semiconductor film, the surfacemorphology of the nitride film becomes favorable in the region above themasks. When the masks formed along this direction are covered with anitride semiconductor film, the growing film forms primarily {1-101}facet planes (FP2) on the masks and then covers them. The {1-101} facetplane is extremely flat, and an edge portion (EP) where the facet planecomes into contact with a crystal growth plane is extremely sharp (seeFIG. 5B). This is probably the reason why the morphology of the surface(FS) of the nitride semiconductor film covering the masks becomesfavorable. If the surface morphology of the underlayer formed of nitridesemiconductor film is good, it becomes possible to reduce the rate ofdefective nitride semiconductor laser device chips having theirrespective ridge stripe portions above the mask A groups.

(Nitride Semiconductor Underlayer)

The underlayer of nitride semiconductor film covering the mask substratemay contain at least one kind of impurity selected from among theimpurity group of Si, O, Cl, S, C, Ge, Zn, Cd, Mg and Be. The followingeffects are obtained with respect to the underlayer of GaN film, AlGaNfilm or InGaN film.

The nitride semiconductor underlayer of GaN film is preferable in thefollowing points. Firstly, since the GaN film is a binary mixed crystal,good controllability of crystal growth can be obtained. Further, thesurface migration length of the GaN film is longer than that of theAlGaN film and shorter than that of the InGaN film, so that lateralcrystal growth can be obtained appropriately to cover the maskscompletely and flatly. Here, the lateral growth refers to the growth ina direction parallel to the main surface of the substrate. Promotion ofthe lateral growth can alleviate the crystal strain in the nitridesemiconductor film covering the masks. The GaN film used as the nitridesemiconductor underlayer has an impurity concentration of preferablygreater than 1×10¹⁷/cm³ and less than 5×10¹⁸/cm³. Addition of theimpurity in this concentration range ensures good surface morphology ofthe nitride semiconductor underlayer and uniform thickness of the lightemitting layer, thereby improving the characteristics of the lightemitting device.

The nitride semiconductor underlayer of AlGaN film is preferable in thefollowing points. When the AlGaN film covers the mask substrate, a voidis less likely to be formed above the mask, so that the probability ofoccurrence of cracks is reduced. The AlGaN film containing Al has thesurface migration length that is shorter than those of the GaN film andthe InGaN film, and this means that the AlGaN film has good adhesivenessto the mask. This is probably the reason why the void is less likely tobe formed above the mask.

According to a result of further investigation of the AlGaN film, the Alcomposition ratio x of the Al_(x)Ga_(1−x)N film is preferably greaterthan 0.01 and less than 0.15, and more preferably greater than 0.01 andless than 0.07. If the Al composition ratio x is smaller than 0.01, itis difficult to suppress occurrence of the void. If the Al compositionratio x is greater than 0.15, the surface migration length describedabove becomes too short (meaning insufficient lateral growth), in whichcase it may be hard to obtain the effect of alleviating the crystalstrain above the mask. The effect similar to that of the AlGaN film canbe obtained with any nitride semiconductor underlayer containing Al.Further, the AlGaN film used as the nitride semiconductor underlayerpreferably has an impurity concentration of greater than 3×10¹⁷/cm³ andless than 5×10¹⁸/cm³. Addition of the impurity in this concentrationrange in addition to Al ensures a properly short surface migrationlength of the nitride semiconductor underlayer.

The nitride semiconductor underlayer of InGaN film is preferable in thefollowing points. In the case that the mask substrate was covered withthe InGaN film, considerable degradation of the lasing lifetimedepending on a position of the formed ridge stripe portion did notoccur. The InGaN film containing In is superior in elasticity to the GaNand AlGaN films. Thus, it is considered that the InGaN film covering themask makes the crystal strain from the nitride semiconductor substratedistributed over the entire nitride semiconductor film, and thusalleviates difference between the crystal strain above the mask and thecrystal strain above the window.

According to a result of further investigation of the InGaN film, the Incomposition ratio x of the In_(x)Ga_(1−x)N film is preferably greaterthan 0.01 and less than 0.18, and more preferably greater than 0.01 andless than 0.1. If the In composition ratio x is smaller than 0.01, theeffect of superior elasticity because of inclusion of In may not beobtained. If the In composition ratio x is greater than 0.18, thecrystallinity of the InGaN film may be degraded. The effect similar tothat of the InGaN film can be obtained with any nitride semiconductorunderlayer containing In. Further, the InGaN film used as the nitridesemiconductor underlayer preferably has an impurity concentration ofgreater than 1×10¹⁷/cm³ and less than 4×10¹⁸/cm³. Addition of theimpurity in this concentration range as well as In is advantageous inthat the surface morphology of the nitride semiconductor underlayerbecomes favorable and the elasticity can also be maintained.

(Thickness of Nitride Semiconductor Underlayer)

To completely cover the mask substrate with the underlayer of nitridesemiconductor film, thickness of the covering film is preferably morethan about 2 μm and less than 30 μm. Here, the covering film thicknessrefers to a thickness which corresponds to the film thickness obtainedwhen a nitride semiconductor film is grown directly on a flat nitridesemiconductor substrate. If the covering film thickness is thinner than2 μm, it becomes difficult to cover the mask substrate completely andflatly with the nitride semiconductor film, though depending upon themask width and the window width on the mask substrate. On the otherhand, if the covering film thickness is thicker than 30 μm, the lasinglifetime becomes a little shorter. This is presumably because growth inthe perpendicular direction (with respect to the main surface of thesubstrate) gradually becomes more prominent than lateral growth on themask substrate, in which case it is difficult to fully obtain the effectof alleviating the crystal strain.

[Second Embodiment]

In a second embodiment, explanation is given for a method of forming anitride semiconductor laser device chip having a ridge stripe structureformed on a filmed mask substrate. The matters not specificallyexplained in the present embodiment are the same as in the firstembodiment described above.

(Method of Forming Filmed Mask Substrate)

Schematic cross sectional views of FIGS. 6A and 6B show a mask substrate101 m including masks formed on a GaN substrate 101 (FIG. 6A) and afilmed mask substrate 100 having an n type Al_(0.05)Ga_(0.95)N film 102covering the mask substrate (FIG. 6B), respectively. The mask substratecan be formed in the following manner.

Firstly, a growth inhibiting film of SiO₂ is formed to a thickness of0.1 μm on a main surface of a (0001) plane of GaN substrate 101, byelectron beam evaporation (EB method) or by sputtering. Thereafter,stripe masks are formed by lithography along the <1-100> direction ofGaN substrate 101. The stripe masks include two kinds of masks, i.e.,masks A (MA) and masks B (MB). The mask A group (MAG) is formed with amask A width (MAW) of 13 μm and a window A width (WAW) of 7 μm. The maskB group (MBG) is formed with a mask B width (MBW) of 5 μm and a window Bwidth (WBW) of 15 μm. Mask substrate 100 m of the present embodiment isthus completed (FIG. 6A).

The mask substrate is subjected to organic cleaning thoroughly, and thentransferred into a MOCVD (metallorganic chemical vapor deposition)system. NH₃ (ammonia) as a source material for the group V element andTMGa (trimethyl gallium) and TMAl (trimethyl aluminum) as sourcematerials for the group III elements are supplied onto the masksubstrate, SiH₄ (Si impurity concentration: 1×10¹⁸/cm³) as an impurityis added to the source materials, and a 6 μm-thick n typeAl_(0.05)Ga_(0.95)N film 102 is grown at a crystal growth temperature of1050° C. The filmed mask substrate 100 of the present embodiment is thuscompleted (FIG. 6B).

The growth inhibiting film may be formed of SiN_(x), Al₂O₃, or TiO₂,besides SiO₂. The longitudinal direction of the stripe masks may bealong the <11-20> direction of GaN substrate 101, instead of the <1-100>direction. Further, although GaN substrate 101 having a main surface ofthe (0001) plane has been used as the nitride semiconductor substrate inthe present embodiment, it is also possible to employ another nitridesemiconductor substrate and another plane orientation. As to theorientation of the main surface of the substrate, a C plane {0001}, an Aplane {11-20}, an R plane {1-102}, an M plane {1-100} or a {1-101} maybe employed preferably. Good surface morphology is obtained with anysubstrate having the main surface with an off angle within 2 degreesfrom any of these plane orientations. As another nitride semiconductorsubstrate, an AlGaN substrate may be employed preferably for example,since it is preferable in the case of a nitride semiconductor laser thata layer having a refractive index lower than that of a clad layer is incontact with the outside of the clad layer to obtain a unimodal verticaltransverse mode.

(Crystal Growth)

FIG. 7 shows a nitride semiconductor laser device chip grown on a filmedmask substrate. This nitride semiconductor laser device includes afilmed mask substrate 100 including masks A and B and an n typeAl_(0.05)Ga_(0.95)N underlayer 102 on a GaN substrate 101, an n typeIn_(0.07)Ga_(0.93)N anti-crack layer 103, an n type Al_(0.1)Ga_(0.9)Nclad layer 104, an n type GaN light guide layer 105, a light emittinglayer 106, a p type Al_(0.2)Ga_(0.8)N carrier block layer 107, a p typeGaN light guide layer 108, a p type Al_(0.1)Ga_(0.9)N clad layer 109, ap type GaN contact layer 110, an n electrode 111, a p electrode 112, aSiO₂ dielectric film 113, and an n type electrode pad 114. Wire bonding(WB) is carried out on p electrode 112 and on n type pad electrode 114.

In formation of this nitride semiconductor laser device, firstly in theMOCVD system, NH₃ as a source material for the group V element and TMGaor TEGa (triethyl gallium) as a source material for the group IIIelement are added with TMIn (trimethyl indium) as a source material forthe group III element and SiH₄ (silane) as an impurity on filmed masksubstrate 100, to grow n type In_(0.07)Ga_(0.93)N anti-crack layer 103to a thickness of 40 nm at a crystal growth temperature of 800° C. Next,the substrate temperature is raised to 1050° C., and TMAl or TEAl(triethyl aluminum) as a source material for the group III element isused to grow n type Al_(0.1)Ga_(0.9)N clad layer 104 (Si impurityconcentration: 1×10¹⁸/cm³) to a thickness of 0.8 μm. N type GaN lightguide layer 105 (Si impurity concentration: 1×10¹⁸/cm³) is then grown toa thickness of 0.1 μm.

Thereafter, the substrate temperature is lowered to 800° C., and lightemitting layer (of multiple quantum well structure) 106 is formedincluding 8 nm thick In_(0.01)Ga_(0.99)N barrier layers and 4 nm thickIn_(0.15)Ga_(0.85)N well layers stacked alternately with each other. Inthe present embodiment, light emitting layer 106 has the multiplequantum well structure starting and ending both with the barrier layers,including 3 quantum well layers (i.e., 3-cycles). The barrier and welllayers are both doped with Si impurity at a concentration of 1×10¹⁸/cm³.A crystal growth break interval of at least one second and at most 180seconds may be provided between growth of the barrier layer and growthof the well layer, or between growth of the well layer and growth of thebarrier layer. Such intervals can improve flatness of the respectivelayers and also decrease the half-width of emission peak in the emissionspectrum.

AsH₃ (arsine) or TBAs (tertiary butyl arsine) may be used to add As inlight emitting layer 106. Similarly, PH₃ (phosphine) or TBP (tertiarybutyl phosphine) may be used to add P, and TMSb (trimethyl antimony) orTESb (triethyl antimony) may be used to add Sb in light emitting layer106. NH₃ as the source material of N may be replaced with dimethylhydrazine in the formation of the light emitting layer.

Next, the substrate temperature is raised again to 1050° C. tosuccessively grow 20 nm thick p type Al0.2Ga_(0.8)N carrier block layer107, 0.1 μm thick p type GaN light guide layer 108, 0.5 μm thick p typeAl_(0.1)Ga_(0.9)N clad layer 109, and 0.1 μm thick p type GaN contactlayer 110. As the p type impurity, Mg (EtCP₂Mg: bisethylcyclopentadienylmagnesium) is added at a concentration from 5×10¹⁹/cm³ to 2×10²⁰/cm³.The p type impurity concentration in p type GaN contact layer 110 ispreferably increased as it approaches the interface with p electrode112. This can reduce the contact resistance at the interface with the pelectrode. Further, oxygen may be added by a minute amount during growthof the p type layers, to remove residual hydrogen in the p type layersthat hinders activation of the p type impurity Mg.

After the growth of p type GaN contact layer 110, the entire gas in thereactor of the MOCVD system is replaced with nitrogen carrier gas andNH₃, and the substrate temperature is decreased at a cooling rate of 60°C./min. Supply of NH₃ is stopped when the substrate temperature isdecreased to 800° C. The substrate is maintained at that temperature forfive minutes, and then cooled to room temperature. The substrate istemporarily held at a temperature preferably in a range from 650° C. to900° C., for a time period preferably in a range from 3 to 10 minutes.The cooling rate to the room temperature is preferably more than 30°C./min. The film thus crystal grown was evaluated by Raman spectroscopy,and it was found that the grown film already had the p typecharacteristic (i.e., Mg had already been activated), even withoutconventional annealing for giving the p type characteristic. The contactresistance was also reduced in formation of p electrode 112. When theconventional annealing for giving the p type characteristic wasadditionally applied, the activation ratio of Mg further improvedfavorably.

In_(0.07)Ga_(0.93)N anti-crack layer 103 of the present embodiment mayhave the In composition ratio of other than 0.07, or the layer itselfmay be omitted. However, the InGaN anti-crack layer is preferablyinserted when crystal lattice mismatch is large between the clad layerand the GaN substrate.

Although light emitting layer 106 of the present embodiment has thestructure starting and ending both with the barrier layers, it may havea structure starting and ending both with the well layers. The number ofwell layers within the light emitting layer is not restricted to 3 asdescribed above. The threshold current density is sufficiently low withat most 10 well layers, enabling continuous lasing at room temperature.In particular, the well layers of at least 2 and at most 6 arepreferable, ensuring the low threshold current density.

While Si has been added in both the well and barrier layers at aconcentration of 1×10¹⁸/cm³ in light emitting layer 106 of the presentembodiment, it may be added to none of the layers. However, the luminousintensity of the light emitting layer is increased when Si is addedtherein. At least one of O, C, Ge, Zn and Mg, besides Si, may beemployed as the impurity to be added in the light emitting layer. Thetotal impurity dose is preferably on the order of 1×10¹⁷/cm³ to1×10¹⁹/cm³. Further, the impurity may be added only in either of thewell and barrier layers, instead of both of the layers.

P type Al_(0.2)Ga_(0.8)N carrier block layer 107 of the presentembodiment may have the Al composition ratio of other than 0.2, or thecarrier block layer itself may be omitted. The threshold currentdensity, however, was lowered with provision of the carrier block layer,because carrier block layer 107 has a function to confine the carriersin light emitting layer 106. The Al composition ratio of the carrierblock layer is preferably set high to enhance the carrier confiningeffect. When the Al composition ratio is set low in the rangeguaranteeing the carrier confinement, mobility of the carriers increasesin the carrier block layer, leading to favorable reduction of electricresistance.

Although Al_(0.1)Ga_(0.9)N crystals have been employed for p type cladlayer 109 and n type clad layer 104 in the present embodiment, the Alcomposition ratio may be other than 0.1. If the Al composition ratio isincreased, differences in energy gap and in refractive index comparedwith light emitting layer 106 increase, so that carriers and light canbe confined in the light emitting layer efficiently, thereby enablingreduction of the lasing threshold current density. On the other hand, ifthe Al composition ratio is lowered in the range ensuring confinement ofcarriers and light, mobility of the carriers in the clad layersincreases, so that an operating voltage of the device can be reduced.

The AlGaN clad layer preferably has a thickness of 0.7-1.5 μm. Thisensures a unimodal vertical transverse mode and increases the lightconfining effect, and further enables improvement in opticalcharacteristics of the laser and reduction of the lasing thresholdcurrent density.

The clad layer is not restricted to the ternary mixed crystal of AlGaN.It may be a quaternary mixed crystal of AlInGaN, AlGaNP, AlGaNAs or thelike. Further, the p type clad layer may have a super-lattice structureformed of p type AlGaN layers and p type GaN layers or a super-latticestructure formed of p type AlGaN layers and p type InGaN layers for thepurpose of reducing its electric resistance.

Although the crystal growth using the MOCVD system has been explained inthe present embodiment, molecular beam epitaxy (MBE), hydride vaporphase epitaxy (HVPE) or the like may also be used for the crystalgrowth.

(Processing into Chips)

The epi-wafer formed in the above-described crystal growth (i.e., waferhaving multiple layers of nitride semiconductor layers epitaxially grownon the filmed mask substrate) is taken out of the MOCVD system andprocessed into laser device chips. Here, the surface of the epi-waferincluding the nitride semiconductor laser device layer is flat, and themasks and windows included in the mask substrate are completely coveredwith the nitride semiconductor underlayer and the light emitting devicestructure layer.

N type Al_(0.05)Ga_(0.95)N film 102 is partly exposed from the frontside of the epi-wafer by dry etching, and then Hf and Al are depositedin this order to form n electrode 111 on the exposed part. N electrodepad 114 of Au is formed on n electrode 111 by evaporation. Ti/Al, Ti/Mo,Hf/Au or the like may also be used as the materials for the n electrode.Hf is preferably used for the n electrode to decrease the contactresistance of the n electrode. The n electrode may be formed on the backside of the mask substrate, since the mask substrate is formed of anitride semiconductor. In this case, the nitride semiconductor substrateshould be doped with an impurity such that it has an n typeconductivity.

The p electrode portion is etched in a stripe manner along alongitudinal direction of the mask, to form a ridge stripe portion (RSin FIG. 7). The ridge stripe portion (RS) having a width of 1.7 μm isformed above mask A (MA), being 2 μm away in the mask width directionfrom the center (MAC in FIG. 7) of the mask A width (MAW). Thereafter,SiO₂ dielectric film 113 is formed by evaporation, and an upper surfaceof p type GaN contact layer 110 is exposed from the dielectric film. Onthe exposed surface of the contact layer, p electrode 112 as stackedlayers of Pd/Mo/Au is formed by evaporation. Stacked layers of Pd/Pt/Au,Pd/Au, Ni/Au or the like may also be used for the p electrode.

Lastly, the epi-wafer is cloven in a direction perpendicular to thelongitudinal direction of the ridge stripe, to form Fabri-Perotresonators of 500 μm each in length. In general, the resonator length ispreferably in a range from 300 μm to 1000 μm. The resonators are formedalong the <1-100> direction of the longitudinal direction of the stripemask. The mirror end surfaces of the resonators correspond to the Mplane {1-100} of the nitride semiconductor crystal. Cleavage forformation of the mirror end surfaces and chip division into laserdevices are carried out with a scriber from the back side of filmed masksubstrate 100. The cleavage is done with the scriber, by scratching notacross the entire back surface of the wafer but only at portions of thewafer, e.g., only the both ends of the wafer. This prevents degradationof sharpness of the end surfaces and also prevents shavings due to thescribing from attaching to the epi-surface, thereby improving the yieldof the devices.

As the feedback method of the laser resonator, commonly known DFB(distributed feedback), DBR (distributed bragg reflector) or the likemay also be employed.

After formation of the mirror end surfaces of the Fabri-Perot resonator,dielectric films of SiO₂ and TiO₂ are alternately formed on one of themirror end surfaces by evaporation, to make a dielectric multilayerreflection film having a reflectance of 70%. Alternatively, multilayerfilms of SiO₂/Al₂O₃ or the like may also be used for the dielectricmultilayer reflection film.

According to cross sectional observation of the nitride semiconductorlaser device chip practically formed in the above-described manner, itwas found that masks A (MA) and masks B (MB) were formed in the samelaser device chip as shown in FIG. 7. Formation of the nitridesemiconductor laser device on the filmed mask substrate according to thepresent invention brought about alleviation of crystal strain andimprovement of lasing lifetime. The probability of occurrence of crackswas also reduced, and the yield of the devices was improved. Thethreshold current density was also decreased by about 3%.

Although the nitride semiconductor laser device having the ridge stripestructure has been explained in the second embodiment, similar effectscan be obtained with a nitride semiconductor laser device having acurrent-blocking layer (see FIG. 8B). Further, the mask A width, thewindow A width, the mask B width and the window B width on the masksubstrate described in the present embodiment may be set to othernumerical values each within the range satisfying the conditionsdescribed above in the first embodiment. The same applies to any of thefollowing embodiments.

[Third Embodiment]

A third embodiment is similar to the first and second embodiments,except that a nitride semiconductor base substrate is employed and thata mask is formed on a nitride semiconductor layer stacked on the nitridesemiconductor base substrate. In a method of forming the filmed masksubstrate in the present embodiment, a GaN base substrate having a mainsurface of a (0001) plane is firstly placed in a MOCVD system. NH₃ andTMGa are supplied on the GaN base substrate, and a low-temperature GaNbuffer layer is formed at a relatively low growth temperature of 550° C.The growth temperature is raised to 1050° C., and NH₃, TMGa and SiH₄ aresupplied on the low-temperature GaN buffer layer to form an n type GaNlayer. The nitride semiconductor substrate having the n type GaN layerformed thereon is then taken out of the MOCVD system.

Thereafter, a growth inhibiting film of SiN_(x) of 0.15 μm thickness isdeposited by sputtering to over the surface of the n type GaN layer onthe substrate taken out of the MOCVD system. Thereafter, stripe masks Aand B of SiN_(x) are formed along the <1-100> direction of the GaNsubstrate by lithography. The mask A width is 10 μm, and the window Awidth is 2 μm. The mask B width is 3 μm, and the window B width is 15μm. The mask substrate of the present embodiment is thus formed.

The mask substrate is subjected to organic cleaning thoroughly, and thentransferred again into the MOCVD system. NH₃ as a source material forthe group V element, TMG as a source material for the group III element,and SiN₄ (Si impurity concentration: 1×10¹⁸/cm³) as an impurity aresupplied on the mask substrate, and a 5 μm thick GaN underlayer isdeposited at a growth temperature of 1050° C. The filmed mask substrateof the present embodiment is thus formed.

The low-temperature GaN buffer layer explained in the present embodimentmay be a low-temperature Al_(x)Ga_(1−x)N buffer layer (0≦x≦1), or thelayer itself may be omitted. With a GaN substrate commercially availableat the present, however, it is preferable to insert the low-temperatureAl_(x)Ga_(1−x)N buffer layer to improve the unfavorable surfacemorphology of the GaN substrate. Here, the low-temperature buffer layerrefers to a buffer layer formed at a relatively low growth temperatureof about 450-600° C. The low-temperature buffer layer formed at a growthtemperature in this range becomes polycrystalline or amorphous.

[Fourth Embodiment]

A fourth embodiment is similar to the first through third embodiments,except that masks of a mask substrate are formed not in stripes arrangedin a single direction, but in a mask pattern including stripe masksarranged in different directions. That is, in the present embodiment,mask substrates having various mask patterns are explained withreference to the schematic top plan views of FIGS. 9A-9C.

The mask substrate of FIG. 9A includes a mask pattern (MP) having stripemasks arranged in two directions perpendicular to each other. This maskpattern is provided at each side of such a mask A group (MAG) asdescribed in the first embodiment and others. Use of such a masksubstrate can improve the crack suppressing effect, and also improve theyield of the light emitting device chips.

The mask substrate of FIG. 9B includes a mask pattern having stripemasks arranged in two directions at an angle of 60° with each other.This mask pattern is provided at each side of such a mask A group asdescribed in the first embodiment and others. These two differentdirections are preferable particularly in the case of using a nitridesemiconductor substrate having a main surface of a {0001} plane, sincethey exhibit characteristics equivalent in crystallography (e.g.,equivalent in the manner of lateral growth). Use of the mask substrateas in FIG. 9B can also improve the crack suppressing effect as well asthe yield of the light emitting device chips.

The mask substrate of FIG. 9C includes a mask pattern having stripemasks arranged in three directions at an angle of 60° with each other.This mask pattern is provided at each side of such a mask A group asdescribed in the first embodiment and others. These three differentdirections are also preferable particularly in the case of using thenitride semiconductor substrate having a main surface of a {0001} plane,since they exhibit characteristics equivalent in crystallography. Use ofthe mask substrate as in FIG. 9C can also improve the crack suppressingeffect and the yield of the light emitting device chips.

[Fifth Embodiment]

In a fifth embodiment, a nitride semiconductor light emitting diodedevice layer is formed on a filmed mask substrate. The light emittingdiode device layer is similarly formed as in a conventional manner,except that a current-constricting portion of its light emitting layeris formed to be included in a region above a mask A group with mask Bgroups being arranged on respective sides of the mask A group. Electriccurrent is introduced through the current-constricting portion whichpractically contributes to light emission.

When the present invention is applied to the nitride semiconductor lightemitting diode device, the luminous intensity is improved. Inparticular, when a light emitting diode device having a short emissionwavelength (less than 420 nm) or long emission wavelength (more than 600nm), such as a nitride semiconductor diode device of white or ambercolor made using nitride semiconductor materials, is formed on thefilmed mask substrate of the present invention, the luminous intensityis increased by more than about 1.5 times compared to the conventionalone. Further, with a conventional nitride semiconductor light emittingdiode device chip, cracks are liable to occur in a current-constrictingportion of the light emitting layer through which a current isintroduced and which practically contributes to light emission. Thecracks are observed as non-light-emitting lines, causing degradation ofluminous intensity and a defective device. According to the presentinvention, the luminous intensity of the nitride light emitting diode isimproved, occurrence of cracks is suppressed, and the rate of defectivedevices is reduced.

[Sixth Embodiment]

A sixth embodiment is similar to the first, second and fifthembodiments, except that at least one substitutional element of As, Pand Sb is included to substitute for some of N atoms in the lightemitting layer. More specifically, at least one substitutional elementof As, P and Sb is contained in substitution for some of N atoms in thewell layer or the barrier layer within the light emitting layer of thenitride semiconductor light emitting device. Here, when the totalcomposition ratio of As, P and/or Sb contained in the well layer isexpressed as x and the composition ratio of N is expressed as y, x issmaller than y and x/(x+y) is less than 0.3 (30%) and preferably lessthan 0.2 (20%). The lower limit of the total concentration of As, Pand/or Sb is preferably greater than 1×10¹⁸/cm³.

This is because, when the composition ratio x of the substitutionalelement is greater than 20%, concentration separation begins togradually occur in which the composition ratios of the substitutionalelement differ in regions within the well layer, and when thecomposition ratio x is greater than 30%, the concentration separationproceeds to crystal system separation into a hexagonal system and acubic system, causing an increased possibility of degradation incrystallinity of the well layer. On the other hand, if the totalconcentration of the substitutional element is smaller than 1×10¹⁸/cm³,it is almost impossible to obtain the effect due to adding thesubstitutional element in the well layer.

The present embodiment has an effect that inclusion of at least onesubstitutional element of As, P and Sb in the well layer reduces theeffective mass of electrons and holes in the well layer, therebyincreasing mobility of the electrons and holes. In the case of asemiconductor laser device, the small effective mass means that carrierpopulation inversion for lasing can be obtained by introducing a smallamount of current. The increased mobility means that, even if electronsand holes in the light emitting layer are lost due to luminousrecombination, electrons and holes can be newly introduced rapidly bydiffusion. That is, according to the present embodiment, it is possibleto obtain a semiconductor laser having a small threshold current densityand exhibiting excellent self-sustained pulsation characteristics(excellent noise characteristics) compared to an InGaN-base nitridesemiconductor laser device of which well layer does not contain any ofAs, P or Sb.

If cracks occur in the nitride semiconductor laser device chip, however,the As, P and Sb elements are liable to escape from the light emittinglayer through the cracks and to diffuse into other layers, making itimpossible to obtain the benefit due to including those elements withinthe light emitting layer.

In the present invention, the rate of occurrence of cracks can bedecreased, while achieving a long lasing lifetime. Accordingly, it ispossible to obtain the above-described benefit of inclusion of the As, Pand Sb elements in at least the well layer within the light emittinglayer of the nitride semiconductor light emitting laser device.

On the other hand, in the case that the present embodiment is applied toa nitride semiconductor light emitting diode device, inclusion of thesubstitutional elements of As, P and/or Sb in the well layer can reducethe In composition ratio within the well layer, compared to the case ofthe nitride semiconductor light emitting diode device including aconventional InGaN well layer. This means that degradation ofcrystallinity due to the In concentration separation can be suppressed.In particular, in the case of the nitride semiconductor light emittingdiode device made using nitride semiconductor materials and having ashort emission wavelength (less than 400 nm) or long emission wavelength(more than 600 nm), the well layer can be formed with a low compositionratio of In or even not containing In at all. As such, the deviceexhibits less color mottling and stronger luminous intensity than aconventional InGaN-base nitride semiconductor light emitting diodedevice. Such benefits of the nitride semiconductor light emitting diodedevice chip are achieved by virtue of the crack suppressing effect ofthe present invention, similarly as in the case of the above-describednitride semiconductor laser device chip.

[Seventh Embodiment]

In a seventh embodiment, a nitride semiconductor laser device in theabove-described embodiments is applied to an optical apparatus. Anitride semiconductor laser device of violet color (with laserwavelength of 360-420 nm) in the above-described embodiments canpreferably be employed in various optical apparatuses, e.g., in anoptical pickup apparatus, from the following view point. Such a nitridesemiconductor laser device operates stably at high temperature (60° C.)and at high output (30 mW), and has a long lasing lifetime, so that itis optimally applicable to a highly reliable high-densityrecording/reproducing optical disk apparatus (shorter laser wavelengthenables recording/reproduction of higher density).

[Eighth Embodiment]

In a eighth embodiment, a nitride semiconductor light emitting diodedevice of the fifth or sixth embodiment is used for a semiconductorlight emitting apparatus. Specifically, the nitride semiconductor lightemitting diode device of the fifth or sixth embodiment is usable as atleast one of light emitting diodes for optical three primary colors(red, green and blue) in a display apparatus (an example of thesemiconductor light emitting apparatus). Use of such a nitridesemiconductor light emitting diode device can realize a displayapparatus suffering less color mottling and exhibiting strong luminousintensity.

Further, such nitride semiconductor light emitting diode devices capableof emitting optical three primary colors can also be used in a whitelight source apparatus. A nitride semiconductor light emitting diodedevice of the present invention having a emission wavelength in a rangeof ultraviolet to purple (about 360 nm to 440 nm) can also be used as awhite light source apparatus by applying fluorescent coating.

Use of such a white light source makes it possible to realize ahigh-luminance backlight consuming less power, in place of a halogenlight source conventionally used for a liquid crystal display. Thiswhite light source can be used as a backlight for a liquid crystaldisplay in a man-machine interface of a portable notebook computer or aportable telephone, realizing a compact and high-definition liquidcrystal display.

[Ninth Embodiment]

According to a ninth embodiment, in a nitride semiconductor laser chipusing a mask substrate including a mask group formed on a partial regionof one main surface of a nitride semiconductor substrate of aconductivity type and having counter electrodes arrangement, acurrent-constricting portion through which electric current isintroduced into the light emitting layer and which substantiallycontributes to lasing is formed above the mask group, thereby realizinga long lasing lifetime and reduction of the threshold voltage. Theeffect of the present embodiment by forming the current-constrictingportion above the mask group of the nitride semiconductor laser chip isobtained only in the case that the substrate included in the masksubstrate of the laser chip is formed of such nitride semiconductor asin the case of the first embodiment.

(Mask Substrate)

The nitride semiconductor laser chip according to the present embodimentshown in schematic cross section in FIG. 13 includes a mask substratehaving a mask group (group of stripe masks formed of growth inhibitingfilm: MG) formed on an n type nitride semiconductor substrate 101, andan underlayer 102 of nitride semiconductor film, n type layers 103-105,a light emitting layer 106, and p type layers 107-110 are successivelycrystal-grown on the mask substrate. It also has a ridge stripe portion(RS). In this nitride semiconductor laser chip, an n electrode 111 isformed on the back side of n type nitride semiconductor substrate 101,and a p electrode 112 is formed on an upper side of a p type layer ofthe ridge stripe portion (RS) (i.e., it has the counter electrodesarrangement). The nitride semiconductor laser chip of FIG. 13 can bemade in a similar manner as will be described later in the tenthembodiment.

The nitride semiconductor laser chip of the present embodiment ischaracterized in that it has the counter electrodes arrangement and inthat the ridge stripe portion (RS) is formed above the mask group (MG)of the mask substrate, as shown in FIG. 13. Here, the width of the maskgroup (MG) from one side edge to another side edge is called a maskgroup width (MGW) (see FIG. 13). The width of the nitride semiconductorlaser chip in a direction which is orthogonal to the longitudinaldirection of the resonator (the direction perpendicular to the plane ofthe drawing of FIG. 13) is called a chip width (CW).

According to a result of the inventors' study, the lasing lifetime tendsto be elongated in the case that the ridge stripe portion (RS) is formedabove one mask (M) within the mask group (MG) included in the nitridesemiconductor laser chip and that the center line (MC) of the mask widthdoes not cross the ridge stripe portion. It has been found throughfurther detailed investigation that the lasing lifetime begins to beelongated considerably in the case that the ridge stripe portion of thenitride semiconductor laser chip is formed above the mask and that adistance from the center of the mask to a side edge of the ridge stripeportion closer thereto is greater than about 1 μm. This is presumablybecause crystal strain of the nitride semiconductor layer region stackedabove the mask is alleviated compared to that of the region stackedabove the window. However, it is considered that, even above the mask,the crystal strain becomes large above the center of the mask.

It has been found that, from the standpoint of a long lasing lifetime,the mask width is preferably greater than 10 μm and less than 20 μm, andmore preferably greater than 13 μm and less than 20 μm in the presentembodiment, similarly as in the case of the mask width A of the firstembodiment.

Further, the lasing lifetime tends to be elongated with a narrower widthbetween the masks (i.e., a narrower width of the window (WP)) in themask group (MG) in the present embodiment, similarly as in the case ofthe window width A of the first embodiment. From the standpoint of along lasing lifetime, the window width is preferably greater than 2 μmand less than 10 μm, and more preferably greater than 2 μm and less than6 μm.

In view of the result of the study about the mask width and window widthas described above, a nitride semiconductor laser chip was formed usinga mask substrate where masks each having a width of greater than 10 μmand less than 20 μm are arranged at intervals of the window width ofgreater than 2 μm and less than 10 μm over the entire surface of anitride semiconductor substrate. The nitride semiconductor laser chiphad the counter electrodes arrangement. With this nitride semiconductorlaser chip, while the lasing lifetime was long, the threshold voltagewas high. Thus, in the present embodiment, a nitride semiconductor laserchip was formed using a mask substrate where a mask group having theabove-described mask width and window width was arranged only in thevicinity of a region beneath the ridge stripe portion. As a result, itwas found that the lasing threshold voltage was decreased by about0.2-0.9 V compared to the case of the nitride semiconductor laser chipusing the mask substrate having the mask group arranged over the entiresubstrate. The decreased threshold voltage further caused elongation ofthe lasing lifetime by about some hundreds of hours compared to theconventional one.

Although the nitride semiconductor laser chip having the ridge stripestructure (see FIG. 8A) as in the case of the first embodiment has beendescribed in the present embodiment, the present invention may of coursebe applicable to a nitride semiconductor laser chip having acurrent-blocking layer (see FIG. 8B).

(Relation Between Coverage Ratio of Mask Group and Threshold Voltage)

Firstly, the coverage ratio of the mask group here refers to apercentage of the mask group width with respect to the chip width shownin FIG. 13. In the graph of FIG. 14, the horizontal axis represents thecoverage ratio (%) of the mask group, and the vertical axis representsthe threshold voltage (V) of the nitride semiconductor laser chip whichhas the counter electrodes arrangement. As shown in FIG. 14, when thecoverage ratio of the mask group decreases from about 50%, the thresholdvoltage begins to decrease (by about 0.2 V compared with the case of thecoverage ratio of 100%). When the coverage ratio is less than about 40%,the threshold voltage decreases to lower than 5 V. When the coverageratio is about 20%, the threshold voltage further decreases.

(Positional Relation Between Mask and Current-Constricting Portion)

As described above in connection with the mask substrate, the lasinglifetime tends to be elongated when the ridge stripe portion (RS) of thenitride semiconductor laser chip is formed above one mask (M) within themask group (MG) and the center line (MC in FIG. 13) of the mask widthdoes not cross the ridge stripe portion. Further, the lasing lifetimebegins to be elongated considerably when the ridge stripe portion of thenitride semiconductor laser chip is formed above the mask and thedistance from the mask center to the closer side edge of the ridgestripe portion is greater than about 1 μm.

Through further detailed investigation about the positional relationbetween the mask and the current-constricting portion, it has been foundthat the rate of defect in which the lasing lifetime of the nitridesemiconductor laser chip becomes shorter than the guaranteed lifedecreases in the case that the current-constricting portion is formed inthe above-described manner above a mask (M) located near the center ofthe mask group width (MGW). Further, the rate of defect in which thethreshold voltage of the nitride semiconductor laser chip becomesgreater than the guaranteed value decreases in the case that thecurrent-constricting portion is formed in the above-described mannerabove a mask located near a side edge of the mask group.

(Longitudinal Direction of Stripe Mask)

In the ninth embodiment also, similarly as in the first embodiment, thelongitudinal direction of the stripe mask formed on the nitridesemiconductor substrate having a main surface of a {0001} C plane ismost preferably in parallel with the <1-100> direction, and nextpreferably in parallel with the <11-20> direction. Even when thelongitudinal direction of the mask made an angle of the order of within±5° to such a specific crystal direction in the {0001} C plane, it didnot cause any substantial effect.

Formation of the masks along the <1-100> direction of the nitridesemiconductor substrate is advantageous in that it causes remarkableeffects of suppressing crystal strain and occurrence of cracks. Thiseffects can reduce the rates of defects (defect with the lasing lifetimeshorter than the guaranteed life and defect due to occurrence of cracks)in the nitride semiconductor laser devices. When the masks formed alongthis direction are covered with a nitride semiconductor film, the filmforms primarily the {11-20} facet planes on the masks to cover them (seeFIG. 5A). The nitride semiconductor film grows from the {11-20} facetplanes, presumably because the {11-20} facet plane (FP1) isperpendicular to the main surface of the nitride semiconductor substrate101 and the mask (M) is formed of a growth inhibiting film suppressingepitaxial growth thereon. Here, the film grows in a direction parallelto the main surface of the substrate (lateral growth), thereby greatlyenhancing the effects of suppressing crystal strain and occurrence ofcracks. By incorporating the feature of the longitudinal <1-100>direction of the masks into the mask substrate, it becomes possible toreduce the rate of defective devices, since the effects of elongatingthe lasing lifetime and suppressing the cracks become more prominent.

Formation of the masks along the <11-20> direction of the nitridesemiconductor substrate is advantageous in that, when the masks arecovered with the nitride semiconductor film, the surface morphology ofthe nitride film becomes favorable in a region above the masks. When adepression shown in FIG. 5B is observed from above the substrate, thenitride semiconductor film grows with the depression hardly meandering.The rates of defects (defect with the lasing lifetime shorter than theguaranteed lifetime and defect with the threshold voltage greater thanthe guaranteed value) in the nitride semiconductor laser devices eachhaving the current-constricting portion formed above the mask substratecan be reduced when the surface morphology of the nitride semiconductorfilm is favorable and the nitride semiconductor film grows with thedepression hardly meandering, presumably because of the followingreasons. When the masks formed along the relevant direction are coveredwith the nitride semiconductor film, the nitride film forms primarily{1-101} facet planes (FP2 ) on the masks to cover them. The {1-101}facet plane is extremely flat, and the edge portion where the facetplane comes into contact with the crystal growth plane is also extremelysharp (see FIG. 5B). These facts are considered to be the reasons whythe surface morphology of the nitride semiconductor film covering themasks becomes favorable.

The above-described masks (or windows) are in the form of stripes, whichis preferable in the following point of view. That is, since thecurrent-constricting portion of the nitride semiconductor laser deviceis generally in the stripe form, it is readily possible to make thecurrent-constricting portion in an optimal position for a longer lasinglifetime when the masks are also in the form of stripes.

(Nitride Semiconductor Underlayer)

As the underlayer of the nitride semiconductor film for covering themask substrate, GaN film, AlGaN film or InGaN film, for example, may beused in the ninth embodiment also, similarly as in the case of the firstembodiment. It is also possible to add at least one impurity selectedfrom the impurity group of Si, O, Cl, S, C, Ge, Zn, Cd, MG and Be in thenitride semiconductor underlayer.

Specifically, in the case of the nitride semiconductor underlayer of GaNfilm, controllability of crystal growth becomes favorable, since the GaNfilm is a binary mixed crystal. Further, the lateral growth of theunderlayer is promoted, possibly alleviating the crystal strain withinthe nitride semiconductor film covering the masks. The GaN film used asthe nitride semiconductor underlayer preferably has an impurityconcentration of greater than 1×10¹⁷/cm³ and less than 8×10¹⁸/cm³.Addition of the impurity in such a concentration range results infavorable surface morphology of the nitride semiconductor underlayer andhence a uniformed thickness of the light emitting layer, so that thedevice characteristics can be improved.

In the case of the nitride semiconductor underlayer of AlGaN film, avoid is less likely to be formed above the mask when the AlGaN filmcovers the mask substrate. The rate of occurrence of cracks is reducedand the lasing lifetime can be improved. Since the surface migrationlength of the AlGaN film is short, the nitride semiconductor filmreadily crystal-grow from the sidewall of the facet plane described inconjunction with FIGS. 5A and 5B. Thus, the lateral growth becomes moreprominent, and the crystal strain is alleviated. As a result, the lasinglifetime can be improved. The Al composition ratio x in theAl_(x)Ga_(1−x)N film is preferably greater than 0.01 and less than 0.15,and more preferably greater than 0.01 and less than 0.07. The AlGaN filmused as the nitride semiconductor underlayer preferably has an impurityconcentration of greater than 3×10¹⁷/cm³ and less than 5×10¹⁸/cm³.Addition of the impurity in this concentration range as well as Aladvantageously shortens the surface migration length of the nitridesemiconductor underlayer, so that the crystal strain can further bealleviated.

In the case of the nitride semiconductor underlayer of InGaN film,difference in lasing lifetime depending on difference in position wherethe current-constricting portion is formed can be made small by coveringthe mask substrate with the InGaN film, and then the rate of defect(defect of the lasing lifetime shorter than the guaranteed lifetime) inthe nitride semiconductor laser chips can be reduced. The In compositionratio x of the In_(x)Ga_(1−x)N film is preferably greater than 0.01 andless than 0.18, and more preferably greater than 0.01 and less than 0.1.The InGaN film used as the nitride semiconductor underlayer preferablyhas an impurity concentration of greater than 1×10¹⁷/cm³ and less than5×10¹⁸/cm³. Addition of the impurity in this concentration range alongwith in results in favorable surface morphology of the nitridesemiconductor underlayer, and elasticity of the underlayer can bemaintained advantageously.

(Thickness of Nitride Semiconductor Underlayer)

The nitride semiconductor underlayer has a thickness of preferablygreater than about 2 μm and less than about 30 μm to completely coverthe mask substrate in the ninth embodiment, similarly as in the case ofthe first embodiment.

[Tenth Embodiment]

In a tenth embodiment, explanation is given for a method of forming anitride semiconductor laser chip having a ridge stripe structure formedon a filmed mask substrate. The matters not specifically explained inthe present embodiment are similar as in the preceding ninth embodiment.

(Method of Forming Filmed Mask Substrate)

In FIG. 11, a mask substrate 101 m including a mask 200 formed on an ntype GaN substrate 101 is shown both in top plan view and crosssectional view. Masks 200 each having a prescribed mask width (MW) arearranged at intervals of a window width (WW). In FIG. 11, the referencecharacter WP represents the window, the characters MC and WC representthe mask center and the window center, respectively, and the arrow (MSD)represents the mask stripe direction.

A schematic cross sectional view of FIG. 12 shows a mask substratehaving masks 200 formed on an n type GaN substrate 101, and a filmedmask substrate 100 having an n type Al_(0.02)Ga_(0.98)N underlayer 102covering the mask substrate. The mask substrate can be formed in thefollowing manner.

Firstly, a growth inhibiting film of SiO₂ is formed to a thickness of0.1 μm on a (0001) main surface of n type GaN substrate 101 by electronbeam evaporation (EB method) or by sputtering. Thereafter, SiO₂ masks200 are formed in stripes along the <1-100> direction of GaN substrate101 by lithography. The stripe masks 200 are formed with a mask width(MW) of 13 μm and a window width (WW) of 7 μm so that three stripe maskscan be included in one nitride semiconductor laser chip. In this case,the mask group width is 53 μm, the chip width is 300 μm, and thecoverage ratio of the mask group is 17.7%. The mask substrate of thepresent embodiment is thus completed.

The mask substrate is subjected to organic cleaning thoroughly, and thentransferred into a MOCVD system. NH₃ as a source material for the groupV element and TMGa and TMAl as source materials for the group IIIelements are supplied on the mask substrate, SiH₄ (Si impurityconcentration: 1×10¹⁸/cm³) as an impurity is further added to the sourcematerials, and then a 25 μm-thick n type Al_(0.02)Ga_(0.98)N underlayer102 is grown at a crystal growth temperature of 1050° C. The filmed masksubstrate 100 of the present embodiment is thus completed (FIG. 12).

The growth inhibiting film may be formed of SiN_(x), Al₂O₃, or TiO₂,besides SiO₂ in the tenth embodiment also, similarly as in the secondembodiment. The longitudinal direction of the stripe mask may be alongthe <11-20> direction of n type GaN substrate 101, instead of the<1-100> direction thereof. Further, although n type GaN substrate 101having a (0001) main surface has been used as the nitride semiconductorsubstrate in the present embodiment, another main surface orientationand another nitride semiconductor substrate may also be employed. As tothe main surface orientation of the substrate, a C plane {0001}, an Aplane {11-20}, an R plane {1-102}, an M plane {1-100} or a {1-101} planemay be employed preferably. Good surface morphology can be obtained withany substrate having a main surface with an off-angle within 2 degreesfrom any of these plane orientations. In the case of a nitridesemiconductor laser, an n type AlGaN substrate, for example, can beemployed preferably as another nitride semiconductor substrate, since itis preferable that a layer having a refractive index lower than that ofa clad layer is in contact with the outside of the clad layer to obtaina unimodal vertical transverse mode.

(Crystal Growth)

FIG. 10 shows a nitride semiconductor laser chip grown on filmed masksubstrate 100. This nitride semiconductor laser chip includes a filmedmask substrate 100 including masks 200 and an n type Al_(0.02)Ga_(0.98)Nunderlayer 102 on an n type GaN substrate 101, an n typeIn_(0.07)Ga_(0.93)N anti-crack layer 103, an n type Al_(0.1)Ga_(0.9)Nclad layer 104, an n type GaN light guide layer 105, a light emittinglayer 106, a p type Al_(0.2)Ga_(0.8)N carrier block layer 107, a p typeGaN light guide layer 108, a p type Al_(0.1)Ga_(0.9)N clad layer 109, ap type GaN contact layer 110, an n electrode 111, a p electrode 112, anda SiO₂ dielectric film 113.

In formation of this nitride semiconductor laser device, firstly in theMOCVD system, NH₃ as a source material for the group V element and TMGaor TEGa as a source material for the group III element with TMIn as asource material for the group III element and SiH₄ as an impurity aresupplied over filmed mask substrate 100, to grow n typeIn_(0.07)Ga_(0.93)N anti-crack layer 103 to a thickness of 40 nm at acrystal growth temperature of 800° C. Next, the substrate temperature israised to 1050° C., and TMAl or TEAl; as a source material for the groupIII element is used to grow n type Al_(0.1)Ga_(0.9)N clad layer 104 (Siimpurity concentration: 1×10¹⁸/cm³) to a thickness of 1.2 μm. N type GaNlight guide layer 105 (Si impurity concentration: 1×10¹⁸/cm³) is thengrown to a thickness of 0.1 μm.

Thereafter, the substrate temperature is lowered to 750° C., to formlight emitting layer (of multiple quantum well structure) 106 including8 nm thick In_(0.01)Ga_(0.99)N barrier layers and 4 nm thickIn_(0.15)Ga_(0.85)N well layers stacked alternately with each other. Inthe present embodiment, light emitting layer 106 has the multiplequantum well structure starting and ending both with the barrier layers,including 3 quantum well layers (i.e., 3-cycles). The barrier and welllayers are both doped with Si impurity at a concentration of 1×10¹⁸/cm³.A crystal growth break interval of at least one second and at most 180seconds may be provided between growth of the barrier layer and growthof the well layer, or between growth of the well layer and growth of thebarrier layer. This can improve flatness of the respective layers andalso decrease the half-width of emission peak in the emission spectrum.

It is needless to say that As, P, and/or Sb may be added to lightemitting layer 106 in the tenth embodiment also, similarly as in thesecond embodiment.

Formation of layers from p type Al_(0.2)Ga_(0.8)N carrier block layer107 to p type GaN contact layer 110 and cooling thereafter to roomtemperature in the tenth embodiment were carried out in a similar manneras in the second embodiment.

The InGaN anti-crack layer 103 of the present embodiment may be omitted,similarly as in the case of the second embodiment. Further, the effectof the number of well layers in light emitting layer 106 and the effectof the added impurity in the present embodiment are similar to those inthe second embodiment.

Still further, p type Al_(0.2)Ga_(0.8)N carrier block layer 107, p typeclad layer 109 and n type clad layer 104 of the present embodiment bringabout similar effects as in the second embodiment if they satisfy therequirements of the second embodiments.

Although the crystal growth using the MOCVD system has been described inthe present embodiment, it is needless to say that an MBE method, anHVPE method and others may also be employed.

(Processing into Chips)

The epi-wafer formed in the above-described crystal growth (i.e., waferhaving multiple layers of nitride semiconductor layers epitaxially grownon the filmed mask substrate) is taken out of the MOCVD system andprocessed into laser device chips. Here, the surface of the epi-waferincluding the nitride semiconductor laser device layer is flat, and themasks and windows included in the mask substrate are completely coveredwith the nitride semiconductor underlayer and the light emitting devicestructure layer.

Hf and Al are deposited in this order to form n electrode 111. Ti/Al,Ti/Mo, Al/Hf/Au or the like may also be used as the materials for the nelectrode. Hf is preferably used for the n electrode to decrease thecontact resistance of the n electrode.

The p electrode portion is etched in a stripe manner along alongitudinal direction of mask 200, to form a ridge stripe portion (seeFIG. 10). The ridge stripe portion (RS) is formed above the mask to havea width of 1.7 μm, at a position 2 μm away in the mask width directionfrom the center of the mask, avoiding a position just above the maskcenter (MC). Thereafter, SiO₂ dielectric film 113 is formed byevaporation, and an upper surface of p type GaN contact layer 110 isexposed from the dielectric film. On the exposed surface of the contactlayer, p electrode 112 as stacked layers of Pd/Mo/Au is formed byevaporation. Stacked layers of Pd/Pt/Au, Pd/Au, Ni/Au or the like mayalso be used for the p electrode.

Lastly, the resonators are formed by cleavage and the reflective filmsare formed on the end surfaces thereof in a similar manner as in thesecond embodiment. The width of the nitride semiconductor laser chipafter chip division was 300 μm.

(Packaging)

For a nitride semiconductor laser chip of high output (greater than 30mW), attention should be paid to measures for heat dissipation. Forexample, a high output laser chip with being preferably junction-down isconnected to a package body by an In soldering material. Alternatively,the high output laser chip may be connected via a sub-mount of Si, AlN,diamond, Mo, CuW, BN, Fe, Cu, Sic, or Au, instead of being directlyattached to the package body or a heat sink portion.

Conclusively, use of the mask substrate according to the presentembodiment brought about improvement of the lasing lifetime, and thelasing threshold voltage was 4.6 V.

As to the mask width, window width and coverage ratio of mask group ofthe mask substrate described in the tenth embodiment, other numericalvalues may be employed under the conditions described in the ninthembodiment. The same applies to the following embodiments.

[Eleventh Embodiment]

An eleventh embodiment is similar to the ninth and tenth embodiments,except that the nitride semiconductor laser chip having a ridge stripestructure as described in the tenth embodiment is changed to a nitridesemiconductor laser chip having a current-blocking structure (FIG. 8B).

The nitride semiconductor laser chip having a current-blocking layer ofthe present embodiment shown in FIG. 15 includes a filmed mask substrate100, an n type In_(0.07)Ga_(0.93)N anti-crack layer 103, an n typeAl_(0.1)Ga_(0.9)N clad layer 104, an n type GaN light guide layer 105, alight emitting layer 106, a p type Al_(0.2)Ga_(0.8)N carrier block layer107, a p type GaN light guide layer 108, a p type Al_(0.1)Ga_(0.9)Nfirst clad layer 109 a, a pair of current-blocking layers 120, a p typeAl_(0.1)Ga_(0.9)N second clad layer 109 b, a p type GaN contact layer110, an n electrode 111, and a p electrode 112.

Current-blocking layer 120 may be any layer that can block electriccurrent introduced from p type electrode 112 to let it pass only througha width (CS) between the pair of current-blocking layers as shown inFIG. 15. For example, an n type Al_(0.25) Ga_(0.75)N layer may be usedas current-blocking layer 120, though the Al composition ratio thereofmay be other than 0.25.

In the eleventh embodiment, similar effects as in the ninth and tenthembodiments can be obtained.

[Twelfth Embodiment]

A twelfth embodiment is similar to the ninth through eleventhembodiments, except that a nitride semiconductor base substrate having apolarity is employed and that a mask is formed on a nitridesemiconductor layer stacked on the nitride semiconductor base substrate.In a method of forming the filmed mask substrate of the presentembodiment, firstly, an n type GaN base substrate having a (0001) mainsurface is placed in a MOCVD system. NH₃ and TMGa are supplied on theGaN base substrate, to form a low-temperature GaN buffer layer at arelatively low growth temperature of 550° C. The growth temperature israised to 1050° C., and NH₃, TMGa and SiH₄ are supplied on thelow-temperature GaN buffer layer to form an n type GaN layer. Thenitride semiconductor substrate having the n type GaN layer formedthereon is then taken out of the MOCVD system.

On a surface of the n type GaN layer of the substrate taken out of theMOCVD system, a growth inhibiting film of SiN_(x) is deposited to athickness of 0.15 μm by sputtering. Thereafter, stripe masks of SiN_(x)are formed along the <1-100> direction of the n type GaN substrate bylithography, with a mask width of 8 μm and a window width of 2 μm. Themasks were formed such that five stripe masks are included in onenitride semiconductor laser chip. In this case, the mask group width was48 μm, the chip width was 250 μm, and the coverage ratio of mask groupwas 19.2%. The mask substrate of the present embodiment is thus formed.

The mask substrate is subjected to organic cleaning thoroughly, and thentransferred again into the MOCVD system. NH₃ as a source material forthe group V element, TMG as a source material for the group III element,and SiN₄ (Si impurity concentration: 1×10¹⁸/cm³) as an impurity aresupplied on the mask substrate, and a 20 μm thick GaN underlayer isdeposited at a growth temperature of 1050° C. The filmed mask substrateof the present embodiment is thus formed. Thereafter, a nitridesemiconductor laser device layer is formed on the filmed mask substratein a similar manner as in the tenth or eleventh embodiment.

The low-temperature GaN buffer layer explained in the present embodimentmay be a low-temperature Al_(x)Ga_(1−x)N buffer layer (0≦x≦1), or thelayer itself may be omitted, similarly as in the third embodiment.

The effects obtained in the twelfth embodiment are similar as in theninth embodiment.

[Thirteenth Embodiment]

A thirteenth embodiment is similar to the ninth through twelfthembodiments, except that a substitutional element of at least one of As,P and Sb is included in the light emitting layer to substitute for someof N atoms. The preferable concentration range and the effects of thesubstitutional elements of As, P and Sb in the present embodiment aresimilar to those of the sixth embodiment. Specifically, in the presentembodiment, it is possible to obtain a semiconductor laser low inthreshold current density and excellent in self-sustained pulsationcharacteristics (noise characteristics) compared to an InGaN-basenitride semiconductor laser device of which light emitting layer doesnot contain any of As, P or Sb. Generally, the lasing lifetime tends tobe elongated as the threshold current density decreases. Thus, in thepresent embodiment, a still longer lasing lifetime can be achieved. Thelaser chip of the present embodiment consumes less power and canoptimally be used for a high output laser, since the threshold voltageis also decreased as the effect of the present invention.

[Fourteenth Embodiment]

In a fourteenth embodiment, a nitride semiconductor laser chip of theabove-described embodiments is applied to an optical apparatus. Anitride semiconductor laser chip of violet color (laser wavelength of360-420 nm) in the above embodiments can preferably be used in variousoptical apparatuses, e.g., in an optical pickup apparatus, for thefollowing reasons. The relevant nitride semiconductor laser deviceoperates stably (or reliably with a long lasing lifetime) at hightemperature (60° C.) and at high output (30 mW), and has a low thresholdvoltage (consuming less power). Thus, it can optimally be used for aportable, high-density recording/reproducing optical disk apparatus(shorter laser wavelength enables recording/reproduction of higherdensity).

In FIG. 16, as an example of a nitride semiconductor laser device of theabove embodiments being used for an optical apparatus, an optical diskapparatus including an optical pickup such as a DVD apparatus is shownin a schematic block diagram. In this optical informationrecording/reproducing apparatus, laser light 3 emitted from a nitridesemiconductor laser device 1 is modulated by an optical modulator 4 inaccordance with input information, and records the information on a disk7 via a scan mirror 5 and a lens 6. Disk 7 is rotated by a motor 8. Atthe time of reproduction, reflected laser light optically modulated bybit arrangement on disk 7 is detected by a detector 10 via a beamsplitter 9, and then a reproduced signal is obtained. The operations ofthe respective elements are controlled by a control circuit 11. Theoutput of laser device 1 is normally on the order of 30 mW uponrecording and on the order of 5 mW upon reproduction.

The laser device according to the present invention can be used not onlyfor the above-described optical disk recording/reproducing apparatus,but also for a laser printer, a bar code reader, a projector includinglasers of optical three primary colors (blue, green, read), and others.

Industrial Applicability

As described above, according to the present invention, it is possibleto improve light-emitting lifetime and luminous intensity of a nitridesemiconductor light emitting device and also to prevent occurrence ofcracks therein.

1. A nitride semiconductor light emitting device chip, comprising: amask substrate (100) including a mask pattern formed on a main surfaceof a nitride semiconductor substrate (101), said mask pattern beingformed of a growth inhibiting film suppressing epitaxial growth of anitride semiconductor layer thereon, and there being a plurality ofwindows unprovided with said growth inhibiting film, there being atleast two different widths as mask widths each between said windowsadjacent to each other, and said mask pattern including a mask A group(MAG) and a mask B group (MBG), and said mask B groups being arranged onrespective sides of said mask A group, and a mask A width in said mask Agroup being set wider than a mask B width in said mask B group; anitride semiconductor underlayer (102) covering said windows and saidmask pattern; and a light emitting device structure having a lightemitting layer (106) including a quantum well layer or a quantum welllayer and a barrier layer in contact with the quantum well layer betweenan n type layer (103-105) and a p type layer (107-110) over saidunderlayer, a current-constricting portion (RS) formed above said maskA, as a region through which substantial electric current is introducedinto said light emitting layer.
 2. The nitride semiconductor lightemitting device chip according to claim 1, wherein saidcurrent-constricting portion is formed more than 2 μm away from thecenter of mask A in its width direction.
 3. The nitride semiconductorlight emitting device chip according to claim 2, wherein saidcurrent-constricting portion is completely included in a region rightabove mask A.
 4. The nitride semiconductor light emitting device chipaccording to claim 2, wherein said current-constricting portion isformed above a region across mask A and window A.
 5. The nitridesemiconductor light emitting device chip according to claim 1, wherein awindow A width in a region of said mask A group is set narrower than awindow B width in a region of said mask B group.
 6. The nitridesemiconductor light emitting device chip according to claim 1, whereinsaid mask A width is in a range of 10-20 μm, and a window A width is ina range of 2-10 μm.
 7. The nitride semiconductor light emitting devicechip according to claim 1, wherein said mask B width is in a range of2-10 μm, and a window B width is in a range of 5-40 μm.
 8. The nitridesemiconductor light emitting device chip according to claim 1, whereinsaid mask A is formed in a stripe having a longitudinal direction inparallel with one of a <1-100> direction and a <11-20> direction of saidnitride semiconductor substrate.
 9. The nitride semiconductor lightemitting device chip according to claim 1, wherein said mask B groupincludes masks B parallel to different directions which form a gridpattern.
 10. The nitride semiconductor light emitting device chipaccording to claim 1, wherein said nitride semiconductor underlayerincludes one of Al_(x)Ga_(1-x)N (0.01≦x≦0.15) and In_(x)Ga_(1-x)N(0.01≦x≦0.18).
 11. The nitride semiconductor light emitting device chipaccording to claim 1, wherein said quantum well layer contains at leastone of As, P and Sb.
 12. The nitride semiconductor light emitting devicechip according to claim 1, wherein said growth inhibiting film is adielectric film.
 13. An optical apparatus comprising the nitridesemiconductor light emitting device chip of claim
 1. 14. A nitridesemiconductor laser chip, comprising: a mask substrate including a maskgroup formed on a region of a main surface of a nitride semiconductorsubstrate (101) having a polarity of n type or p type, said mask groupincluding a plurality of masks (200) and a plurality of windows arrangedalternately, each said mask being formed of a growth inhibiting filmsuppressing epitaxial growth of a nitride semiconductor layer thereon,and each said window being unprovided with said growth inhibiting film;a nitride semiconductor underlayer (102) having a polarity and coveringthe entire region of said mask substrate including said mask group; anda light emitting device structure having a light emitting layer (106)including a quantum well layer or a quantum well layer and a bafflerlayer in contact with the quantum well layer between an n type layer(103-105) and a p type layer (107-110) over said underlayer, acurrent-constricting portion (RS) formed above said mask group, as aregion through which substantial electric current is introduced intosaid light emitting layer, electrodes (111, 112) being formed on a backside of said mask substrate and a front side of said light emittingdevice structure, respectively, and a width (MGW) of said mask groupbeing narrower than a width (CW) of the laser chip in a directionorthogonal to a length direction of a laser resonator.
 15. The nitridesemiconductor laser chip according to claim 14, wherein in the widthdirection orthogonal to the length direction of the resonator of saidlaser chip, the width of said mask group is less than 50% of a width ofsaid laser chip.
 16. The nitride semiconductor laser chip according toclaim 14, wherein said mask has width in a range of 10-20 μm, and saidwindow has a width in a range of 2-10 μm.
 17. The nitride semiconductorlaser chip according to claim 14, wherein said current-constrictingportion is formed above the mask located approximately at the center ofsaid mask group.
 18. The nitride semiconductor laser chip according toclaim 14, wherein said current-constricting portion is formed above themask located near a side edge of said mask group.
 19. The nitridesemiconductor laser chip according to claim 14, wherein said nitridesemiconductor underlayer has a thickness of greater than 2 μm and lessthan 30 μm.
 20. The nitride semiconductor laser chip according to claim14, wherein said nitride semiconductor underlayer is GaN which containsat least one impurity selected from an impurity group of Si, O, Cl, S,C, Ge, Zn, Cd, Mg and Be in a range between 1×10¹⁷/cm³ and 8×10¹⁸/cm³.21. The nitride semiconductor laser chip according to claim 14, whereinsaid nitride semiconductor underlayer includes one of Al_(x)Ga_(1-x)N(0.01≦x≦0.15) and In_(x)Ga_(1-x)N (0.01≦x≦0.18).
 22. The nitridesemiconductor laser chip according to claim 21, wherein an impurityconcentration contained in said Al_(x)Ga_(1-x)N (0.01≦x≦0.15) is greaterthan 3×10¹⁷/cm³ and less than 8×10¹⁸/cm³, and an impurity concentrationcontained in said In_(x)Ga_(1-x)N (0.01≦x≦0.18) is greater than1×10¹⁷/cm³ and less than 5×10¹⁸/cm³.
 23. The nitride semiconductor laserchip according to claim 14, wherein said quantum well layer contains atleast one of As, P and Sb.
 24. An optical apparatus comprising thenitride semiconductor laser chip of claim 14.